1. Field of the Invention
The present invention relates to a phase retarder film whose durability is improved, and a liquid crystal display device using the same.
2. Related Prior Art
U.S. Pat. No. 5,430,566 discloses a phase retarder film wherein the in-plane refractive index is different from the refractive index in the thickness direction. Since a crystalline layer unit of an layered inorganic compound used in this phase retarder film has a stratified structure, when the film is formed by using the layered inorganic compound alone or in combination with a resin on a transparent resin substrate, the crystalline layer units are oriented parallel to the surface of the transparent resin substrate, and randomly oriented in the plane direction. As a result, this phase retarder film has optical characteristics that the in-plane refractive index is larger than the refractive index in the thickness direction, and exhibits optical characteristics that the in-plane retardation value is smaller than the retardation value in the film thickness direction.
A layered inorganic compound layer formed on an isotropic transparent resin substrate has optical characteristics that the in-plane retardation value is approximately 0 nm and the retardation value in the thickness direction is larger than that. The retardation value in the thickness direction [R'=(nXY-nZ).multidot.d (nXY: average in-plane refractive index, nZ: refractive index in the thickness direction, d: thickness of the film)] as a parameter of the optical characteristics can be easily controlled to a predetermined value, and a uniform phase retarder film having large area can be produced. This phase retarder film can be used for improving the viewing angle characteristics of a vertically aligned nematic (VAN) type liquid crystal display device and a 90 degrees twisted nematic (TN) type liquid crystal display device.
It is also possible to control a ratio (R40/R0) of the retardation value measured by inclining by 40 degrees around the slow axis to the in-plane retardation value (R0) within the range from 0.9 to 1.1 by a method of forming a layered inorganic compound layer directly on a uniaxially oriented film having a negative birefringent anisotropy, such as polystyrene, as a transparent resin substrate, or a method of forming a lavered inorganic compound layer on an isotropic transparent resin substrate, followed by laminating with a uniaxially oriented film having a negative birefringent anisotropy, such as polystyrene, by using an adhesive. By using these retardation films, it is also possible to improve the viewing angle characteristics of a super-twisted nematic (STN) type liquid crystal display device.
Furthermore, it is possible to control R40/R0 within the range larger than 1.1 by a method of forming a layered inorganic compound layer directly on a uniaxially oriented film having a positive birefringent anisotropy, such as polycarbonate, as a transparent resin substrate, or a method of forming a layered inorganic compound layer on an isotropic transparent resin substrate, followed by laminating with a uniaxially oriented film having a positive birefringent anisotropy, such as polycarbonate, by using an adhesive. By using these retardation films, it is also possible to improve the viewing angle characteristics of an optically compensated birefringent (OCB) type liquid crystal display devices such as a bend aligned nematic (.pi. cell) type liquid crystal display device, a hybrid aligned nematic (HAN) liquid crystal display device and the like.
In the production of this phase retarder film, a water-soluble resin and a layered inorganic compound are preferably used in combination in view of the formability of the layer comprising a layered inorganic compound on a transparent resin substrate, exhibition of optical characteristics, mechanical characteristics and the like. Particularly, a method of applying a water dispersion containing polyvinyl alcohol or a derivative thereof and a water-swellable layered inorganic compound on a transparent resin substrate to form a film is preferably used.
A phase retarder film produced by applying a water dispersion containing polyvinyl alcohol or a derivative thereof and a water-swellable layered inorganic compound on a transparent resin substrate disclosed in U.S. Pat. No. 5,430,566 is superior in optical characteristics. However, there sometimes arises a durability failure in high temperature and high humidity environment in a state of interposing the phase retarder film between a polarizing film and a liquid crystal cell and laminating by using an adhesive, that is, the layer of polyvinyl alcohol or a derivative thereof and a water-swellable layered inorganic compound is peeled off from the transparent resin plate.
It is considered that this durability failure occurs in the following manner. That is, since a water-soluble resin and a hydrophilic compound such as water-swellable layered inorganic compound are mainly used, they absorb a large amount of water in high humidity environment to cause volume expansion larger than that of the transparent resin substrate and, at the same time, deterioration of adhesion at the interface due to softening of the water-soluble resin occurs.
As a phase retarder using a layered inorganic compound, there is known a plate having a suitable thickness made of a mica crystal by utilizing the cleavage property thereof, wherein the in-plane retardation value is adjusted to .lambda./4, as described in Handbook of Optics (W. G. Driscoll et al., Optical Society of America, McGraw-hill, USA, 1978), page 10-108, sections 57, 58. However, since the cleavage property of the crystal is utilized, it is difficult to obtain a uniform plate having wide area, which can be used in the liquid crystal display device. Since the plate is composed of a single crystal and a ratio of the in-plane retardation value (R) to the retardation value (R') in the thickness direction is constant, R is uniquely decided when R' is designed to an arbitrary value. Furthermore, R exceeds 50 nm when R' becomes large such as several hundreds nm. That is, R' can not be controlled optionally by maintaining R at a small value.
Under these circumstances, the present inventors have studied intensively. As a result, there could be developed a phase retarder film which has excellent durability, wide area and uniform optical characteristics, wherein the in-plane retardation value is smaller than the retardation value in the film thickness direction, by using an organic clay compound which can be dispersed in an organic solvent. Thus, the present invention has been accomplished.